Conventionally, melted metal powder is sprayed onto the friction surfaces of cam followers and rocker arms for motor vehicle engines to form hard metal layers. The molten metal forced out from the spray nozzle spreads out in a conical form and a large amount of the metal is wasted, since the surface of the work to be treated is small. However, because the stream of the molten metal jetted out in a conical shape from the nozzle differs in metal density between the central portion and the outer peripheral portion of the stream, it is impossible to form a hard metal layer of uniform thickness on the friction surface of the work, if the stream from the nozzle is constricted. Accordingly it has been practiced to direct the central portion only of the spread-out stream toward the friction surface of the work, without allowing the outer peripheral portion of the stream to impinge on the friction surface. Japanese Patent Specification for Objection SHO. No. 45-32607 and Japanese Patent Specification for Public Inspection SHO. No. 50-106828 disclose an apparatus in which only the work surface to be treated is adapted to be exposed with the other portion covered with a mask plate. The apparatus includes conveyor means for carrying works as arranged thereon in one or two rows with their desired work surfaces positioned close to each other, a gas burner disposed below the path of travel of the work surfaces, and a metal spray nozzle and a fusing nozzle provided above the path of travel, such that a large number of works are continuously treated while being passed below the spray nozzle. With the apparatus disclosed, the mask is placed directly on the work surface and is exposed to the metal stream from the spray nozzle, with the result that sprayed metal is deposited on the mask surface in the form of a thick layer. During the spraying operation, therefore, the mask must be periodically replaced to remove the deposited metal layer from the mask surface.